During the manufacture of aspherical lenses or lenses which are not rotationally symmetric, such as lenses having a toric surface or free-form surfaces, tools or polishing heads are normally used which are smaller than the lens surface to be machined, so called zonal polishing tools.
During the machining of selected zones, the tool or the polishing head is guided over the surface while a polishing agent is applied to those areas of the lens surface to be machined which are not covered by the tool, which polishing agent is then incorporated between the polishing tool and the lens surface to be machined, thus improving the polishing performance. To adapt the polishing head to the shape of the surface to be machined, the polishing head is mounted so as to be tiltable by means of a ball-and-socket joint. Furthermore, the polishing head comprises an elastic carrier layer for the polishing film so that the polishing head can be locally modified in shape for adaptation to the shape of the lens.
Such a polishing tool is known from DE 10 2004 062 319 B3. This publication describes a polishing device for optical lenses comprising a holder having an axis of rotation X and which is intended for attachment to a polishing machine, and a sleeve which is attached to said holder and serves to rotationally drive a tool holder or polishing head holder which is attached to said sleeve, which tool holder is guided so as to be tiltable by means of a guiding piston which is coaxially mounted in said holder and can be displaced in the direction of the axis of rotation X.
As mentioned above, the polishing agent is applied to the uncovered parts of the surface to be polished during machining of selected zones. An additional application of polishing agent is not necessary at first.
For machining spherical surfaces, especially in precision optics where mineral glass lenses are normally used, large polishing tools are used which cover the entire surface to be polished in order to obtain a highly precise spherical surface. The polishing agent cannot be supplied as described above in this case because the surface to be polished is not exposed during the polishing process. In this case, the polishing agent is advantageously supplied in a different way.
A device for supplying a polishing agent for precision machining of spherical surfaces is known from DE 199 05 583 B4. Said device comprises a cast shaping tool having a spherical machining surface for receiving a polishing film (not shown), and including a supply channel which is open towards the machining surface, and a rotatable connecting portion having a connection channel and by means of which the tool is detachably mounted. A supply part is connected to the connecting portion in the radial direction and can be rotated relative to said connecting portion, wherein there is sufficient play between the connecting portion and the supply part to ensure the leakage of auxiliary agents. The rotary joint created in this way is mounted on the outer circumference of a base part of the spindle and carries the polishing tool. Neither the rotary joint nor the polishing tool can be exchanged easily.
The relative angular position between the tool comprising the machining surface and the surface to be machined is fixed. Neither the tool nor the base part can be pivoted.
The polishing tool which is known from the state of the art is not designed as a polishing plate, but as a rigid casting to which a polishing film is attached. A polishing plate as used herein is characterized by a carrier part, usually made of plastic, to which an elastic foam layer is applied. Said foam layer serves as a carrier layer for a polishing film, which is preferably exchangeable and is also called polishing pad.
The elastic foam layer enables the polishing film to be adapted to the local conditions of the surface to be polished, in addition to the tilting movement of the polishing plate by means of the ball-and-socket joint. As explained, aspherical surfaces are mostly polished whose curvature varies in the radial direction as well as in the circumferential direction.